The differences are if someone else is accessing the same device. Nice,
but it don't change the basic functionality.

Exactly what device name to use though, is depending on which port he is
using, and where the OS is mapping it to. And of course, if right mknod
incantations have been done (hopefully by running MAKEDEV).

It is important to understand that all the "device names" in /dev are
just filenames, and can be renamed as much as you like. The names there
have no correspondence with the actual devices they map to. It's just
convenient to have device names that appears to have a mapping to the
physical device they connect you to.

So, assuming he just runs MAKEDEV, and lets that script worry about the
names of the things in /dev, I would guess that information about what
the kernel prints at boot time in relation to this would help.
And then, of course, it will also depend on wether cu will behave nicely
if the person have a cable where not all signals are connected, and so on...

Now I have a netbsd laptop and I would like to login to the first
netbsd machine *without* the usb-to-serial cable, since the laptop has
a serial port. I read about cu and tip program, but I was not able to
use them. My question more precisely is: which device in /dev/ should
I pass to cu or tip. I tried the following commands with no success:
# cu -l /dev/tty00 -s 9600
# cu -l /dev/tty01 -s 9600
# cu -l /dev/dty00 -s 9600
# cu -l /dev/dty01 -s 9600

Define "no success". What is the error message? What does your dmesg
say about serial ports?
You must use dty* (dial-out) devices.
On my macppc I use the following to connect to my landisk using puc(4)
serial:
$ cu -l /dev/dty11 -s 9600
Also see my bin/36060 for issues with devices and cu(1) permissions
vs. group "dialer".
SY, Uwe